1. Field of the Invention
The present invention relates to a process for applying fat to a granulated product which includes combining in a container a predetermined quantity of the granulated product with a predetermined quantity of a fat-containing liquid to moisten the granulated product, generating a predetermined negative pressure in the container of the moistened granulated product, and releasing the negative pressure over a predetermined duration while mixing the moistened granulated product. The present invention also relates to a device for applying fat to a granulated product by combining in a container the granulated product with a fat-containing liquid to moisten the granulated product; generating a predetermined negative pressure in the container of the moistened granulated product; and releasing the negative pressure over a predetermined duration while mixing the moistened granulated product.
2. Discussion of Background Information
Throughout this document the term "applying fat", being the opposite of the term "removing fat", is used analogously to "applying moisture" and the opposite, "removing moisture". In the case of applying moisture, a liquid is brought into contact with a product in such a way that the liquid does not just penetrate the surface but also the interior of the product. Conversely, in the case of removing moisture, the liquid is not just removed from the surface of the product but also withdrawn from the interior, thus drying the product.
In the same sense, in the case of applying fat, a product is brought together with a fat-containing liquid, in such a way that the fat-containing liquid not only remains on the surface but also penetrates into the interior.
There is a problem when granulated food products, i.e. product elements such as extruded products, pellets, expander pellets etc. with a high fat-content, are produced. This is noted with fish food which is intended to float on the surface of the water. If the addition of fat takes place prior to the cooking process or forming process, then the mechanical cohesion properties of the finished product are poor, resulting in a poor consistency of the formed product element.
If on the other hand, the fat is applied after the forming process by spray drums or mixers in the conventional way, then there is a problem in that only the surface of the product to which fat is to be applied, absorbs the fat-containing liquid, or that the fat-containing liquid penetrates the product only slowly by capillary action. In the case of increased viscosity or closed or small pores, this process takes place even more slowly.
For the above-mentioned reasons, the quantity of fat which can be applied to the product by spray drums or mixers, must be limited to between 5 and 10% of the granulate quantity.
The Japanese patent application with publication number HEI3-180 163 (1991) shows and describes an improved system where a product is admitted, by a dosing lock, to an upward-pointing feed screw. An oil pump with a predetermined level is maintained in the lower part of the inclined feed screw, into which said sump the product falls and is moistened during the upward transport. In addition, a vacuum pump ensures that there is negative pressure in the feed screw. In other words, during upward transport, the product transported upward after exposure to the fat-containing liquid is subjected to a predefined negative pressure.
In this system, the level of the oil sump can be set and varied.
According to details provided, the aforementioned rate of oil absorption of between 5 and 10% of the granulate quantity is improved to 13.5% of the granulate quantity in the above-mentioned system.
However, practical application shows that still higher fat absorption is desired. It is thus the object of the invention to increase such absorption, of the fat-containing liquid, in the granulated product. This is particularly desirable for fish food. According to the invention, this object is met by the combination of features disclosed herein.
Fish food formulations which usually contain a considerable portion of fish meal are gently extruded in an extruder at temperatures around 120.degree. C. with a water content of 20 to 30%. With optimal extrusion conditions, this plastic material expands in the extruder at temperatures exceeding 100.degree. C. as a result of the associated steam pressure. Subsequently, on leaving the extruder, the material solidifies to a bread-like, foamy structure, i.e., the moisture is removed, and after experiencing the associated cooling, the solidified product is cooled.